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法律学包含不存在于表的多个列的 MySQL 查询
可以使用多个 AS 关键词轻松添加查询中不存在的多个列。
我们先创建一个表。创建表的查询如下所示 −
mysql> create table ColumnDoesNotExists -> ( -> UserId int, -> UserName varchar(20) -> ); Query OK, 0 rows affected (0.67 sec)
使用 insert 命令在表中插入一些记录。查询如下所示 −
mysql> insert into ColumnDoesNotExists(UserId,UserName) values(100,'Larry'); Query OK, 1 row affected (0.14 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(101,'Sam'); Query OK, 1 row affected (0.22 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(102,'Mike'); Query OK, 1 row affected (0.15 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(103,'David'); Query OK, 1 row affected (0.15 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(104,'Robert'); Query OK, 1 row affected (0.10 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(105,'Maxwell'); Query OK, 1 row affected (0.20 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(106,'Bob'); Query OK, 1 row affected (0.17 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(107,'John'); Query OK, 1 row affected (0.17 sec) mysql> insert into ColumnDoesNotExists(UserId,UserName) values(108,'James'); Query OK, 1 row affected (0.18 sec)
示例
使用 select 语句显示表中的所有记录。查询如下所示 −
mysql> select *from ColumnDoesNotExists;
输出
+--------+----------+ | UserId | UserName | +--------+----------+ | 100 | Larry | | 101 | Sam | | 102 | Mike | | 103 | David | | 104 | Robert | | 105 | Maxwell | | 106 | Bob | | 107 | John | | 108 | James | +--------+----------+ 9 rows in set (0.00 sec)
示例
以下是添加查询中不存在的列名的查询 −
mysql> select UserId,UserName,23 AS Age from ColumnDoesNotExists;
输出
+--------+----------+-----+ | UserId | UserName | Age | +--------+----------+-----+ | 100 | Larry | 23 | | 101 | Sam | 23 | | 102 | Mike | 23 | | 103 | David | 23 | | 104 | Robert | 23 | | 105 | Maxwell | 23 | | 106 | Bob | 23 | | 107 | John | 23 | | 108 | James | 23 | +--------+----------+-----+ 9 rows in set (0.00 sec)
示例
现在让我们看看一次添加多个列的查询。此处我们添加“Marks”和“CountryName”列 −
mysql> select UserId,UserName,23 AS Age,99 AS Marks,'UK' AS CountryName from ColumnDoesNotExists;
输出
+--------+----------+-----+-------+-------------+ | UserId | UserName | Age | Marks | CountryName | +--------+----------+-----+-------+-------------+ | 100 | Larry | 23 | 99 | UK | | 101 | Sam | 23 | 99 | UK | | 102 | Mike | 23 | 99 | UK | | 103 | David | 23 | 99 | UK | | 104 | Robert | 23 | 99 | UK | | 105 | Maxwell | 23 | 99 | UK | | 106 | Bob | 23 | 99 | UK | | 107 | John | 23 | 99 | UK | | 108 | James | 23 | 99 | UK | +--------+----------+-----+-------+-------------+ 9 rows in set (0.00 sec)
更新于:26-6-2020
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